1. Field of the Invention
The present invention relates to assembly assist mechanisms and, more particularly, relates to a low inertia, low-stiffness method and apparatus for passively inserting a part such as a gear into an assembly such as a gear set.
2. Discussion of the Related Art
Manual assembly tasks, particularly those involving the handling of heavy parts, are often the source of repetitive motion injuries and quality control problems. Flexible automation solutions for a variety of assembly tasks have already been developed and some have been implemented in production. However, many of the automated solutions have proven to be too unreliable for use in production. Tolerances within the assembled parts and variability in part location have prevented sufficiently high success rates.
For example, when inserting a splined gear into a planetary gear set, a significant amount of uncertainty exists as to the relative positions of the gears as a whole, as well as the orientation of the meshing teeth. In some cases, the uncertainty in relative positioning exceeds the clearances between the features at the properly mated position. For example, in the case of gear meshing, the uncertainty of location in one set of gear teeth relative to the other may exceed the clearances between the gear teeth. As a result, the positioning capability of a robot is not sufficient to reliably compensate for the positioning uncertainty. For example, the alignment of gear teeth on two gears can be a very difficult task due to location uncertainty even though the directional alignment of the gears"" centerlines are well within the clearance requirements.
Previous robotic assembly assist mechanisms have been proposed that attempt to alleviate some of these problems. Such mechanisms typically are rigidly coupled to a robot arm and to a tool support element that is coupled to a gripper that receives a gear to be installed. The mechanisms may further be configured to permit limited movement of the tool support element relative to the robot arm to accommodate some misalignment of the gear relative to the robot arm. Mechanisms of this type are said to be xe2x80x9ccompliantxe2x80x9d because the tool holder has some freedom of motion relative to the mechanism. Compliance in a given direction reduces the contact force in that direction. However, heretofore-available mechanisms have not been sufficiently compliant with a sufficient number of degrees of freedom to maintain contact and friction forces acceptably low throughout the gear insertion process under all possible insertion conditions. As a result, these insertion mechanisms tend to be unreliable and can risk gear jamming and/or damage to the assembled parts or the gear insertion mechanism. These risks heretofore have been alleviated only by employing relatively complex active controls in the form of feedback control of the robot throughout the insertion process, thereby adding cost and complexity to the system.
It is therefore a first object of the present invention to provide a low inertia, compliant robotic insertion assist mechanism that is operable under control of a controller for inserting a part into an assembly.
It is a second object of the invention to provide a compliant robotic insertion mechanism that reduces the forces necessary to insert parts into assemblies.
It is a third object of the invention to provide a compliant robotic insertion mechanism that has a sufficient number of degrees of freedom relative to the controller to maximize the reliability of the assembly operation and to reduce the forces and corresponding stresses at the point of insertion.
In accordance with a first aspect of the invention, these objects are achieved by providing an assembly assist mechanism configured for the passive insertion of a part such as a gear into an assembly such as a gear set. The insertion mechanism includes a mounting element that is configured for connection to a controller. An output element is in mechanical communication with the mounting element at one end, and is connected to a tool holder at a second, opposite end. The tool holder receives the part that is to be inserted into the assembly. The mechanism is configured to permit vertical movement of the part generally along the z axis relative to the mounting element and independently of rotation of the part about the z axis so as to permit passive insertion of the part into the assembly under low contact force conditions.
It is another object to provide a method for inserting a gear into a gear set using the gear insertion mechanism in accordance with the first object of the invention.
In accordance with a second aspect of the invention, this object is achieved by moving a controller such as a robot arm into a position such that a gear to be inserted is generally aligned with the corresponding gear set. The controller is preprogrammed to lower the arm, thereby also lowering the gear a given distance in the vertical or z direction until the gear contacts the gear set and the insertion mechanism compresses to xe2x80x9cprimexe2x80x9d the gear for insertion into the gear set. The controller then undergoes a series of rotational iterations to rotate the gear back and forth about the z axis until the teeth on the gear are in meshing alignment with the teeth of the mating gear(s) of the gear set, at which time the gear will passively drop by gravity (and possibly spring biasing forces) into meshing engagement with the mating gear(s) of the gear set. The controller need not directly apply a vertical force to the gear as it is inserted into the gear set. The resulting relatively low forces and consequent low-inertia and low-stiffness insertion minimize the risk that the parts will become damaged during assembly.
Due to the nature of the connection between the components of the insertion mechanism, the gear to be installed may be passively translatable during this insertion process in at least the z direction and in an x-y plane to ensure that it is properly aligned with the gear set and that gear insertion takes place under low inertia and with a minimum of friction. In order to minimize contact and friction forces in applications in which the gear(s) which mate with the gear to be inserted are relatively massive, the insertion mechanism may further permit the gear to passively rotate about the z axis to accommodate limited rotation between the input element of the gear insertion mechanism and the inserted gear and to thereby negate or reduce the need for the gear to drive the mating gear(s) of the gear set. The insertion process requires no active feedback to the controller for its implementation. At most, a limit switch or the like may generate a signal at the end of the process to signify that the gear has been successfully inserted into the gear set.